Fixes .calc-grid is undefined & .make-grid is undefined.

LessPHP thinks that there is no suitable mixin candidate when @index reaches 0 because none of the existing mixin conditions matches it. That is why it is returning errors like `.calc-grid is undefined` and `.make-grid is undefined`.

The temporary solution is to add these 2 empty mixins to "mixins.less" and those errors should go away.